A long standing problem with drill bits is a phenomenon known as balling. The cutters on the drill bit shear the rock as the bit is rotated. As a result of such a shearing action, a cutting is generated that is comprised of formation particles encapsulated by fines from the drilling fluid. This encapsulation creates a filter cake that results in a differential pressure between the interior and exterior of the cutting thereby giving the cutting structural strength. This gives the cutting both strength and ductility thereby making the cutting difficult to weaken and clear from the cutting elements. In addition, the cuttings when under such pressure have an affinity for the bit surface adjacent to the cutters. In a PDC bit the cuttings tend to accumulate in the junk slots between blades. This accumulation leads to a phenomenon known as balling that occurs when a sufficient volume of cuttings have accumulated to cut off the fluid flow out of the junk slot. This can then lead to a situation where the cuttings are being extruded out the junk slot due to the high forces exerted on the drill bit rather than the preferred scenario where they are evacuated by the drilling fluid. It has been shown that balling even in a single junk slot on a 6 bladed PDC bit can reduce the rate of penetration (ROP) by as much as 80%.
The drilling mud is normally circulated through a bit body and exits at nozzle locations between adjacent blades. Prior designs tended to point those nozzles toward the hole bottom due to limitations imposed by manufacturing. This technique cleans the junk slots of drill cuttings in varying degrees, depending on a host of factors including, but not limited to, the formation being drilled, the rate of penetration, the mud system in use, and various design aspects of the PDC bit. More recent developments in PDC bits have attempted to vary the angle of the fluid jet from the nozzles to about 45 degrees away from the vertical bit axis. Such a design is shown in U.S. Pat. No. 6,164,394. Even earlier a company called British Bits advertised a lateral stream from a nozzle directed radially between blades. Yet other designs for an impregnated diamond bit featured flow channels for cooling and cuttings removal with the hope that radial flow would turn 90 degrees and take cuttings between the teeth. This design is shown in U.S. Pat. No. 3,938,599. Other designs of laterally oriented nozzles are shown in WO 97/07913.
While turning the nozzles away from the axial orientation toward the hole bottom may have provided some incremental reduction in bit balling, the results were difficult to quantify. One thing that the lateral orientation standing alone did not address is how to get the cutting to release from the bit surface as it is produced at the cutter. Rather than letting the bit surface contact the cutting right after it is made at the cutter, the present invention takes the approach that there is a benefit to spacing the surface of the bit away from the region where the cutting is formed. There is a further benefit in orienting the spray of a nozzle behind the cutting before it engages the bit surface. One way this is done in the preferred embodiment is to dispose a trough adjacent the cutters so as to make the bit surface recede as the cutting is formed and at the same time orient the spray in the trough to provide fluid energy to keep the newly formed cutting away from the bit surface and propelling it radially into the junk slot. These and other features of the present invention will be more apparent to those skilled in the art from a review of the preferred embodiment and the associated drawings that appear below, while recognizing that the full scope of the invention is to be found in the literal and equivalent scope of the claims.